1. Field of the Invention
The invention relates to a method of manufacturing a semiconductor structure in which a first dopant is introduced into a first surface part of a monocrystalline semiconductor substrate region so as to form at least a first buried layer of a first conductivity type, after which an epitaxial layer is grown on the substrate region and, by diffusion from the first buried layer, the overlying part of the epitaxial layer is converted throughout its thickness into a first region of the first conductivity type.
2. Description of the Prior Art
In manufacturing semiconductor devices, such as monolithic integrated circuits, it is often necessary to form juxtaposed regions of opposite conductivity types adjoining the same semiconductor surface. For example, in circuits comprising complementary insulated gate field effect transistors, the n-channel and the p-channel transistors are provided in juxtaposed regions of different conductivity types. In practice these regions are obtained starting from a substrate region of a first conductivity type in which one field effect transistor is formed. In this substrate region, a region of the second opposite conductivity type is formed by doping at the location intended for another field effect transistor complementary to the first.
This doping may be carried out, for example, by diffusion from the surface in which case the doping concentration generally decreases from the surface. In many cases this is an undesirable doping profile. Such a profile gives rise either to a high threshold voltage for the associated field effect transistor as a result of a high surface doping, or, when this surface doping is chosen low, to a high resistance of the diffused region parallel to the surface. This latter may give rise to undesired phenomena, for example thyristor effects, and the like.
Effort has been made to vary the doping profile in the desired direction in various manners. According to a first method, after the indiffusion, the doping concentration at the surface can be reduced by outdiffusion in an evacuated capsule. However, this is a complicated and expensive process step. In the formation of the second region by ion implantation, a certain quantity of a dopant of the first conductivity type may also be implanted in addition to a dopant of the second conductivity type, so that a lower net doping concentration is obtained at the surface by compensation. However, this is disadvantageous because the charge carrier mobility decreases as a result of the high total doping concentration (donors plus acceptors).
A method as described in the opening paragraph is disclosed in British Patent Specification No. 1,176,263. In this method, a region of the first conductivity type extending up to the surface of the epitaxial layer is formed in an epitaxial layer of the opposite conductivity type by diffusion from a buried layer of the first conductivity type formed between the substrate and the epitaxial layer. The region thus formed has a doping concentration which decreases from the substrate towards the surface. However, the lateral diffusion gives rise to an important lateral expansion of the region, which increases the required crystal area and decreases the packing density of the circuit. In order to reach a maximum packing density, and sometimes also for other reasons, it is desirable to form juxtaposed regions of opposite conductivity types which form a p-n junction with each other which is substantially perpendicular to the semiconductor surface or at least has a direction which is not determined entirely by lateral diffusion from one of the two regions.